Electrical Installation in Compressor Systems

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To dimension and install a compressor requires knowledge of how component parts affect each other and which regulations and provisions apply. The following is an overview of the parameters that should be considered to obtain a compressor installation that functions satisfactorily with regard to the electrical system.

The motor protection class is regulated by standards. The dust and water jet-resistant design (IP55) is preferred over open motors (IP23), which may require regular disassembly and cleaning. In other cases, dust deposits in the machine will eventually cause overheating, resulting in shortened service life. Since the compressor package enclosure provides a first line protection from dust and water, a protection class below IP55 may also be used.

The motor, usually fan-cooled, is selected to work at a maximum ambient temperature of 40˚C and an altitude of up to 1000 m. Some manufacturers offer standard motors with maximum ambient temperature capability of 46°C. At higher temperatures or higher altitude, the output must be derated. The motor is usually flange-mounted and directly connected to the compressor. The speed is adapted to the type of compressor, but in practice, only 2-pole or 4-pole motors with respective speeds of 3,000 rpm. The rated output of the motor is also determined (at 1,500 rpm).

The rated output of the motor is also determined by the compressor, and should be as close to the compressor's requirement as possible. A motor that is over dimensioned is more expensive, requires an unnecessarily high starting current, requires larger fuses, has a low power factor and provides somewhat inferior efficiency. A motor that is too small for the installation in which it is used is soon overloaded and is consequently at risk for breakdowns.

The starting method should also be included as a parameter when selecting a motor. The motor is only started with a third of its normal starting torque for a star/delta–start. Therefore, a comparison of the motor and compressor torque curves may be useful to guarantee proper compressor starts.

The three different motor starting methods

The most common starting methods are direct start, star/delta–start and soft start. Direct start is simple and only requires a contactor and overload protection. The disadvantage it presents is its the high starting current, which is 6–10 times the motor's rated current, and its high starting torque, which may, for example, damage shafts and couplings. The star/delta–start is used to limit the starting current. The starter consists of three contactors, overload protection and a timer. The motor is started with the star connection and after a set time (when the speed has reached 90% of the rated speed), the timer switches the contactors so that the motor is delta-connected, which is the operating mode.

The star/delta–start reduces the starting current to approximately 1/3 as compared to the direct start. However, at the same time, the starting torque also drops to 1/3. The relatively low starting torque means that the motor's load should be low during the starting phase, so that the motor virtually reaches its rated speed before switching to the delta connection. If the speed is too low, a current/ torque peak as large as with a direct start will be generated when switching to the delta connection.

Soft start (or gradual start), which can be an alternative start method to the star/delta–start, is a starter composed of semiconductors (IGBT-type power switches) instead of mechanical contactors. The start is gradual and the starting current is limited to approximately three times the rated current.

The starters for direct start and star/delta–start are, in most cases, integrated in the compressor. For a large compressor plant, the units may be placed separately in the switchgear, due to space requirements, heat development and access for service. A starter for soft start is usually positioned separately, next to the compressor, due to heat radiation, but may be integrated inside the compressor package, provided the cooling system has been properly secured. High-voltage powered compressors always have their start equipment in a separate electrical cabinet.

Control voltage

No separate control voltage is usually connected to the compressor, as most compressors are fitted with an integrated control transformer. The transformer's primary end is connected to the compressor's power supply. This arrangement offers more reliable operation. In the event of disturbances in the power supply, the compressor will be stopped immediately and prevented from restarting. This function, with one internally-fed control voltage, should be used in situations in which the starter is located at a distance from the compressor.

Short-circuit protection

Short- circuit protection, which is placed on one of the cables' starting points, can include fuses or a circuit breaker. Regardless of the solution you select, if it is correctly matched to the system, it will provide the proper level of protection. Both methods present advantages and disadvantages. Fuses are well-known and work better than a circuit-breaker for large short-circuit currents, but they do not create a fully-isolating break, and have long tripping times for small fault currents. A circuit-breaker creates a quick and fully-isolating break, even for small fault currents, but demands more work during the planning stage, as compared to fuses. Dimensioning short-circuit protection is based on the expected load, but also on the limitations of the starter unit.

For starter short-circuit protection, see the IEC (International Electrotechnical Commission) standard 60947-4-1 Type 1 & Type 2. The selection of Type 1 or Type 2 is based on how a short-circuit will affect the starter.

Type 1: "… under short circuit conditions, the contactor or starter shall cause no danger to per-sons or installation and may not be suitable for further service without repair and replacement of parts."

Type 2: "… under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and shall be suitable for further use. The risk of light welding of the contactors is recognized, in which case the manufacturer shall indicate the maintenance measures …"

Cables

Cables shall, according to the provisions of the standard, "be dimensioned so that during normal operations they do not experience excessive temperatures and that they shall not be damaged thermally or mechanically by an electric short circuit". The dimensioning and selection of cables is based on the load, allowed voltage drop, routing method (on a rack, wall etc.) and ambient temperature. Fuses can be used, for example, to protect the cables and can be used for both short-circuit protection and overload protection. For motor operations, short-circuit protection is used (e.g. fuses) as well as separate overload protection (usually the motor protection included in the starter).

Overload protection protects the motor and motor cables by tripping and breaking the starter when the load current exceeds a preset value. The short- circuit protection protects the starter, overload protection and the cables. Cable dimensioning accounting for load is set out in IEC 60364-5-52. An additional parameter must be kept in mind when dimensioning cables and short-circuit protection: the "tripping condition". This condition signifies that the installation must be designed so that a short-circuit anywhere in the installation will result in quick and safe breaking. Whether the condition is met is determined by, among other things, the short-circuit protection and the cable length and cross-section.

Phase compensation for heavily loaded transformers

The electric motor does not only consume active power, which can be converted into mechanical work, but also reactive power, which is needed for the motor's magnetization. The reactive power loads the cables and transformer. The relationship between the active and reactive power is determined by the power factor, cos φ. This is usually between 0.7 and 0.9, where the lower value refers to small motors.

The power factor can be raised to virtually 1 by generating the reactive power directly by the machine using a capacitor. This reduces the need for drawing reactive power from the mains. The reason behind phase compensation is that the power supplier may charge for drawing reactive power over a predetermined level, and that heavily loaded transformers and cables need to be off-loaded.

Installing a compressor system is easier than it used to be. There are still a few things to keep in mind though, most importantly where to place the compressor and how to organise the room around the compressor. Learn more here.